What to do if CNN cannot overfit a training set on adding dropout?
Sorry if this is a bad use of answer to add comment but since my reputation is not high enough this is only way to leave a comment to OP's question.
I think some of the answers misunderstood the OP's intention.
Over fitting is used as a means to test the complexity of the model - if a model cannot overfit a small dataset then it's likely not able to generalize well.
It's not that OP misunderstood the meaning of over fitting.
For Instance, I think this discussion is relevant: https://stats.stackexchange.com/questions/492165/what-to-do-when-a-neural-network-cannot-overfit-one-training-sample